There are known a large number of organic molecules each reversibly changing its molecular structure upon light irradiation. Such organic molecules, each of which exhibits a significant color change due to the molecular structure change, are called photochromic molecules.
A photochromic molecule reversibly changes its molecular structure and physicochemical property through a photochemical reaction (also referred to as “photochromic reaction”). There has thus been a lot of research on the photochromic molecule as an optical switching unit that is incorporated in various molecules to cause chemical and physical changes. There has been reported, as such an optical switching unit, a metal complex including a photochromic molecule as a ligand.
In a metal complex, the light emission property of a metal ion depends on symmetry of the metal complex. It is known that in a metal complex including a photochromic molecule as a ligand, a change in molecular structure of the photochromic molecule changes the symmetry of the metal complex and thus changes the light emission property of the metal ion. In other words, irradiating the photochromic molecule in the metal complex with light changes the molecular structure of the photochromic molecule, which in turn changes the light emission property, for example, an emission intensity of the metal ion which emission intensity is observed when the metal ion is excited by exciting light particular for the metal ion. This indicates that a metal complex including a photochromic molecule as a ligand makes it possible to increase and attenuate the emission intensity with use of an optical signal.
An example of a metal complex including a photochromic molecule as a ligand is a complex illustrated in FIG. 6, the complex including, incorporated in phenanthroline as a ligand, diarylethene serving as a photochromic molecule. This complex can undergo an emission color change with use of MLCT transition control of the complex by ON/OFF switching of π conjugated system through a photochromic reaction. The above complex is thus reported to have a possibility in application as a molecular device (see Non Patent Literature 1).
There has also been reported reversible fluorescence switching, as illustrated in FIG. 7, that involves a photochromic site in phthalocyanine and that uses a fluorescence quenching effect caused by a photochromic reaction (see Non Patent Literature 2).
There has further been reported a photochromic property of bis(terpyridine) metal complex conjugated with azobenzene (see Non Patent Literature 3).
There have also been reports, although not on a metal complex including a photochromic molecule as a ligand, but that concern a coloring material and an optical information storage material each including diarylethene serving as a photochromic molecule (see Patent Literatures 1, 2, and 3).
Patent Literature 1 discloses an optical information storage material that, in order to improve stability in information storage and durability for repeated use, includes in a main recording layer including a photochromic composition a pigment which absorbs light at a wavelength longer than an absorption wavelength of a photochromic material that is in a colored state. Patent Literature 2 discloses an optical information storage material that is capable of emitting, for example, brown on its own and that has a high speed of interconversion between tautomers. The respective optical information storage materials of Patent Literatures 1 and 2 each achieve optical recording monomolecularly, and are thus not concerned with a metal complex including a photochromic molecule as a ligand.
Patent Literature 3 attempts to provide a solution to the problem that it is difficult to improve sensitivity of a color dosimeter including a photochromic compound because the color dosimeter transmits radioactive rays. Specifically, Patent Literature 3 discloses a method for effectively introducing in a film a metal serving as a material that does not transmit radioactive rays, in which method a metal complex is present at a particular proportion with respect to the photochromic compound. Patent Literature 3, however, does not intend to utilize a change in the light emission property of a metal ion which change is caused by a coordinate bond of a photochromic molecule to a metal.